X-ray fluorescence analysis (XRF analysis) is a powerful analytical method for detecting and characterizing very different materials. Depending on the analytic problem, different variants of XRF analysis are used; these differ in terms of the measurement geometry. An important variant that is used very frequently is the wavelength-dispersive XRF analysis, which uses the Bragg condition for analyzing the x-ray radiation.
The Bragg condition is a fundamental law of physics, which is applied for measuring the photon energy of x-ray radiation. If such radiation is diffracted at a crystal lattice, the following relationship applies between crystal and radiation parameters: λ=2d·sin ϑ
λ: wavelength of the radiation
2d-value: lattice plane distance of the crystal (property of the lattice structure)
ϑ: angle of reflection with respect to the crystal lattice plane
Here, x-ray spectrometers for wavelength-dispersive XRF analysis, as a rule, have a measurement chamber with a goniometer.
Goniometers according to the prior art require rotational movements that are matched to one another of two coaxial axes, namely for a crystal and for a detector unit, such that the Bragg condition is satisfied in a reproducible manner between the active crystal (or a multilayer) and the detector system. In order to be able to analyze different wavelength regions, crystal changers with a plurality of crystals, which are adjustable in a motor driven or manual manner such that they may be set into a working position, are also used in such designs.
Here, important boundary conditions are the following:
A high accuracy is required. This relates, in particular, to the angular position of the two spindles of the arms of the goniometer relative to one another and to the spatial orientation of the spindles relative to the x-ray-optical components, such as e.g. masks and collimators, but also, for example, relative to the crystal surface.
The effects of deformations of the measurement chamber, which is strained after the evacuation by the pressure difference between the varying atmospheric pressure and the vacuum in the measurement chamber, on the geometry of the beam path must be limited by structural means.
In respect of miniaturizing the overall appliances, a light compact, but nevertheless torsionally rigid construction is particularly important.
Thermal influxes of the goniometer and temperature variations in the measurement chamber should be as low as possible. By way of example, this has negative effects on the analyzer crystal. An important analyzer crystal consists of pentaerythritol (PET), which has a very pronounced coefficient of thermal expansion. There is a correspondingly pronounced change in the 2d value of the crystal structure in the case of temperature variations, as a result of which there is also a corresponding change in the reflection angle ϑ for x-ray radiation at a wavelength λ. Therefore, pronounced temperature variations lead to an incorrect measurement result.
Since an accurate measurement of the intensities in the case of low-energy x-rays, such as e.g. x-ray fluorescence radiation of light elements, is not possible in air because the x-rays are absorbed or scattered too strongly by gases in the air, such goniometers can only be housed in a vacuum chamber. As a result of the pressure difference between the varying atmospheric pressure and the vacuum, known apparatuses are complicated and, in particular, designed with thick walls in order to ensure the stability of the beam paths.
In an x-ray spectrometer, an x-ray source irradiates the sample to be analyzed. The x-ray fluorescence emitted by the sample enters into the evacuated measurement chamber, is incident on an analyzer crystal and reflected onto an x-ray detector from the latter. The crystal and detector are placed by means of a goniometer in such a way that the Bragg condition is satisfied for the wavelength to be analyzed.
Conventional goniometers typically contain stepper motors or servomotors which drive the spindles of the goniometer by means of appropriate gearing. Since the beam paths of an XRF goniometer must lie in a vacuum, two fundamental options emerge for such conventional drive concepts:
1. Motors Outside of the Vacuum Chamber
Here, the gearing mechanism may be housed within the vacuum chamber. However, to this end, only a small selection of greases are available for the gearing mechanism on account of the vacuum conditions and heat that arises in the gearing mechanism can only be dissipated poorly, which in turn leads to interferences from the set up in the vacuum chamber. However, the gearing mechanism may also lie outside of the vacuum chamber—like in the case of the Bruker S8 Tiger, published at https://www.bruker.com/de/products/x-ray-diffraction-and-elemental-analysis/x-ray-fluorescence/s8-tiger/technical-details.html. However, a disadvantage in that case is that a large opening is required for the passage of the shaft into the vacuum chamber, said large opening receiving rotatable parts and, at the same time, needing to be vacuum tight. Consequently, a massive structure of the gearing mechanism holder, and hence of the wall of the measurement chamber, is required on account of the pressure difference.
2. Motors within the Vacuum Chamber
However, it is desirable to attach the motors within the vacuum chamber in order to avoid massive setups for the vacuum passage and improve the vacuum tightness. Here too, the heat dissipation by way of thermal radiation is poor. Cooling can only be achieved by complicated measures, such as e.g. water cooling or the like.
Thermal Power Losses During Static Holding of a Position:
Motors for goniometers are usually designed as stepper motors which are actuated within the scope of micro-step operation in order to improve the resolution. In this mode, the motor phases must permanently be supplied with a power at a specific ratio in order not to fall back to the next full step of the stator.
This property is independent of the connected gearing mechanism type. As a result, electric power is converted into heat within the vacuum chamber.
Servomotors must likewise be permanently supplied with power in order to hold a non-balanced spindle in a static position. This also applies to specific direct drives, which act without a gearing mechanism directly onto the goniometer spindles.
The use of piezo-motors for goniometers for x-ray diffractometers is known from JP 2002 311199 A, but not for applications in conjunction with a vacuum housing for the measurement chamber for the purposes of analyzing x-ray fluorescence radiation as in the case of a generic x-ray spectrometer. In JP 2002 311199 A, two ring-shaped piezo-motors are described in a horizontal geometry for driving two coaxial shafts. The piezo-motors have been installed in symmetric fashion and have the same dimensions. U.S. Pat. No. 9,008,272 B2 also exhibits an x-ray spectrometer with movable arms that are adjustable by piezo-motors.